Separation of low molecular weight hydrocarbons from petroleum mixtures



May 23, `1950 J. A. PAT-reason 2,509,044 SEPARATION 0F L0' IOLECULAR WEIGHT yNYDROGARBONS s mou mmm ux'runss Filed July 3. 1947 2 Shoots-Shut 1 John. G Patterson. .nverztor May 23, 1950 J. A. PATTERSON 2,509,044

sEPAEATxoN oF Low MOLECULAR WEIGHT mRocARBoNs mou PETROLEUM mx'ruEEs Filed July 3, 1947 2 Sheets-Sheet 2 unnfwud 011:00 10.1 A 'l' Am )U H m ..1 In 0e rlllllhlhvlllul' I @d IIII m ...11| AI .unoullll f/ o 0.,.. 1 J .nl an :IT o@ HHHHH a I I I I l E HHH Fr *'.I

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John. C1. Patterson. Urn/enter' Clbbor'ne bess.

Patented May 23, 1950 SEPARATION F LOW MOLECULAR WEIGHT HYDROCABBON S FROM PETROLEUM MIX- TUBES John A. Patterson, Beverly, Standard Oil Development ration oi' Delaware N. J., assigner to Company, a corpo- Application July 3, 1947, Serial No. 758,844

3 Claims.

This invention relates to the concentration and separation of low molecular weight hydrocarbons from complex mixtures and in particular to the separation and purification of ethylene by processing involving a modified distillation double tower.

Variation of pressure has been employed in the prior art as a distillation aid in the fractionation of mixtures. Air fractionation is a particular development in this eld, depending also on a unique heat exchanger system to reduce the cost of refrigeration. The double tower used in air fractionation involves not only the division of the distillation zone into two pressure sections, but also the separation of the sections by a common heat exchanger unit. The base of the `lower pressure section of the zone functions as a reboiler in that section for the bottoms supplied from the high pressure section and as a top condenser for the more volatile constituents of the feed supply furnished to the higher pressure section. To render the unit compact and to minimize refrigeration losses, the lower pressure section is commonly super-imposed upon the higher pressure but the two sections may be arranged as otherwise suitable provided there is a common heat exchanger or condenser-reboiler system.

The common heat exchanger in the double tower functions in a dual capacity, namely, to furnish the heat for fractionation in the lower pressure zone and also to supply the reflux for the high pressure zone. The effectiveness of the heat exchange unit is made possible by reason of the inversion of the temperature caused by the pressure chosen for the two sections of the distillation column. In air fractionation, the difference in pressure between the two fractionating dones causes the normal temperature differential of the boiling points of oxygen and nitrogen to be reversed so that the high pressure nitrogen condenses at a higher temperature than that required to boil the low pressure oxygen. This inversion of temperature is only advantageously effected, however, with mixtures of narrow boiling range, such as air and is not generally applicable to more complex mixtures such as the low molecular weight hydrocarbon mixtures available in oil fields and petroleum refineries because of the wide range of the critical constants of the component compounds.

' The present invention is concerned with modifications in the normal processing employing the double tower. The modifications are necessary in order that mixtures of wide boiling range be accommodated and suitably fractionated. vIn the conventional double tower, the presence of the higher boiling hydrocarbons in the feed results in a relatively high bottoms temperature in the low pressure section. The pressure in the higher pressure section of the double tower is determined by the boiling point of the overhead liquid withdrawn from thatsection which is supplied as reux to the lower pressure section. Thus, the presence of the high boiling compounds in the feed, and thus in 'turn'in the low pressure section, requires a corresponding increase in temperature and consequently in pressure in the high pressure section. Such high pressure makes operation of the unit ineicient and expensive due to compression requirements and to the fact that separation of constituents is more diiilcult at higher pressures. Above a definite limiting concentration of high boiling constituents, processing in the conventional double tower arrangement becomes unworkable since the critical temperatures of the hydrocarbons at the top of the high pressure section are exceeded.

Processing according to the present invention permits the separation of the high boiling constituents from such mixtures of wide boiling range, without foregoing the advantages of processing in the double tower. Thus, in one method of processing according to the invention, the feed stock is supplied to the low pressure section of the double tower system and the residue is passed to the higher pressure section. In the high pressure distillation section a light fraction is separated as a condensate in the upper portion from a heavy residual fraction removed from the base of the section. This modification of processing necessitates the use of a pump to transfer the bottoms from the low pressure section to the high pressure section where this residue stream acts as feed to the high pressure section. As in the conventional double tower, maximum advantage is taken of the heat exchange between the streams within the system for eiiicient operation. By such processing the need for a high temperature in the bottom reboiler portion of the lower pressure section on account of high boiling material in the feed supply is obviated.

Another method of processing according to the invention involves supplying the feed stock to the high pressure section as in normal double tower processing and dividing the high boiling materials thus obtained in the high pressure section into two streams, namely, a distillation residue and a side stream. The side stream is supplied as the feed to the low pressure distillation section while the residual fraction is removed from the 3 system at the bottom.` In this manner the desired lower boiling hydrocarbons such as ethylene and propylene are separated from the heavier C: and C4 hydrocarbons of cracked propane stock by reason of the separation of these lower boiling compounds in the side stream from the much higher boiling compounds removed inthe distillation residue from the high pressure section.

Thus, the applicants invention involves processing in which a high temperature ing-the reboiler at the base of the low pressure section is not necessarily required in'spite of the presence the-feed to the system of high boiling constituents. l'Ihe applicants processing essentially inv voivos mintoimm; o boumgmixtureoof voiotiuty intermediate between those ofthe condensate-in the high pressure zone and the high boiling con-- stituents separated as distillation residue from the bottom of the high pressure section. This is achieved by'obviating the need for isolating the high boiling constituents in the low'pressure section in the two principal embodiments of the invention. This is accomplished by 1) in separate- 1y removingfrom the feed in the high pressure section the highest honing constituents and 2).

in making the initial separation of the lightest constituents in the low pressure zone and supplying to the high pressure section the thus reduced feed for closer separation of the hight boiling constituents.

In order that the invention may be more fully understood, the following descriptions of processing together with the attached drawings, Figures 1, 2 and 3, are presented. In Figure 1 diagram is presented in'which processing is carried out by supplying the feed stock to the low pressure section of a double tower. In Figures 2 and 3, two flow diagrams are presented as a comparison of processing by supplying the feed stock to the high pressure section and obtaining the two high boiling fractions therefrom (Figure 2) with normal type processing in the double tower (Figure 3). In these drawings, the lower pressure section is shown as being super-imposed upon the higher pressure section but the two sections may be otherwise relatively located provided connection occurs in a common heat transfer or condenser-reboiler unit. h

In Figure 1, a hydrocarbon mixture is supplied through line I0. In this specic illustration, the feed stock has the following composition:

per cent Ethylene 42.9 Ethane 17.9 Propylene 26.6 Propane 11.0 CH- 1.6

Lv the feed supply. before entering the low pressure section, also passes through a heat exchanger I8. In the high pressure section il an average pressure of about 100 pounds per square inch gauge is maintained, while in the upper low pressure section an average pressure of about pounds per square inch gauge is maintained. Also,

in the high pressure section 'the number of plates is 20, and in the lower pressure section the num- 5 boiling materials as obtained from a previous operation' as distillation from the low pressure section are pumped into the higher pressure -section and distillation carried outlinV order to supply the heat necessary fcrcarrying out the 1o distillation in the low pressure section. With the feed stock of composition as described under about l'pounds per square inch pressure, fractionation is obtained by maintaining a temperature of about 90 F. in the low pressure reboiler section 20 and an overhead vapor exit line 22 temperature of about 135 F.

The distillation residue from the loy pressure section is removed through line 24, passed through heat exchanger i8 and thence vthrough pump 26 and into the high pressure section through line 28. The pump 2B increases the pressure upon the material supplied to the high pressure section through line 28 to about 100 pounds per square inch. With a reboiler l2 temperature maintained at about 25 F. and an upper condensing eilect in rthe reboiler 20, a vapor temperature in the upper portion of the distillation I zone i4 is maintained at about 50 F. Under such conditions of processing, the distillation residue has a composition as follows:

Per cent Ethylene 3.9 Ethane 25.0 Propylene 48.2 Propane 20.0

A portion of the condensate from section I4 is removed through line 30 and passed through cooler 32 and thence as reflux through line 34 to the low pressure section I6. In passing through line 30 and the cooler 32, the pressure is reduced to about 10 pounds per square inch.

Through line 22, the overhead vapors from the distillation zone IB are first passed through the coolers 32 and I8, respectively. The product obtained is of the following composition:

Per cent Ethylene 90.0 Ethane 9.9 Propylene 0.1

Another manner of processing according to the invention is shown in the flow diagram,

ventional double tower processing shown in Figure 3. In the processings shown in Figures 2 and 3, illustrations are presented with regard to the treatment of a feed stock of the following o compositions:

- Per cent Methane 0.4 Ethylene 38.4 Ethane 16.0

u Propylene 27.5 Propane 12.0 04+ 5.7

The f eed is supplied through line 50, through cooler 52 and into about the middle of the high pressure section 5l of the double tower, the low pressure section being indicated by the numeral 56. The temperature of the feed supplied to the tower is about 10 F. The pressure in the distillation zone 54 is maintained at about 100 pounds per square inch,A while in the lower pres- Fgure 2, and comparison made with the consure section the pressure is maintained at about pounds per square inch. With the feed supply as given, the temperature of F. is maintained in the lower portion of the distillation zone 64 by means of the heat exchanger 50. An upper vapor temperature of about 55 F. is maintained as a result of the condensing eii'ect due to the condenser-reboiler 60 of the dividing portion of the double tower. In the high pressure section 54, about plates are maintained; and in the low pressure section 50, about 20 plates. From a lower plate P, at a temperature of 5 F., a side stream is removed through line 62, passed through heat exchanger 04, and thence through line 8B into the lower half of the low pressure distillation zone 56. From the bottom of the high pressure zone, distillation residue is removed through line 68 and passed to other equipment for further processing. The composition of this residue is as follows:

From the upper portion of the high pressure distillation zone 54, condensate is removed through line 10 and passed through cooler 'l2 and thence through line 'E4 into the upper portion of the low pressure distillation zone l."Ii as reux.

With the condenser-reboiler 60 maintaining a condensing temperature of 55 F. in the high pressure section, the reboiler eiect in the low pressure section gives a temperature of 65 F. With such a temperature, the overhead vapor exit temperature is 130 F. Under such conditions of processing, the overhead vapors are removed through line 16 and passed through heat exchangers 12 and 04. The composition of the product is:

Per cent Methane 0.9 Ethylene 85.1 Ethane 13.8 Propylene 0.2

The distillation residue is removed from the low pressure section through line 18 and thence through coolers 64r and 52: The composition of this product is as follows:

In contrast to such processing, with the same feed stock supplied through line |00 of Figure 3 and through cooler |02 at a temperature of about 10 F., the feed is supplied to the tower in the lower half of the high pressure section |04, the lower pressure section of the tower being indicated by the reference numeral |00. For advantageous separation of the constituents, it has been-found that a pressure of 150 pounds per square inch is required in the high pressure section |04 and a bottoms temperature of 0 F. is maintained by the reboiler |08. Under such conditions of temperature and pressure, the condensing vapors in the upper portion have a temperature of 60 F. The distillation residue is removed from the distillation zone |04 through line ||0, passes through heat exchanger H2 and thence through line 4 to the lower part oi the distillation zone |00. From the upper portion of Per cent Methane 0.9 Ethylene 85.1 Ethane 13.8 Propylene 0.2

From the lower portion of the distillation zone 20 los, the bottoms are removed through une m, passed through heat exchangers ||2 and |02. The composition of this product is as follows:

Per cent 25 Ethylene 3.3 Ethane 17.7 Propylene 48.0 Propane 21.0 04+ 10.0

It is also evident that in the conventional type processing considerable portion of ethylene is removed in the final residue in low concentration, while in the iinal residue from the processing according to the invention no ethylene content occurs. Also, in the residue from the low pressure section of the processing according to the invention a higher content of ethylene is obtained than in the residue from the processing involving the use of the conventional double tower. Advantages also with the processing according to the invention are that a lower pressure is maintained in the higher pressure section and that less of a cooling effect is necessary to maintain the bottoms temperature in the higher pressure section of the tower involved in the processing according to the invention.

What is claimed is:

1. In a fractionation process for separating ethane and ethylene in a predominantly Ca to C4 hydrocarbon fraction in a unitary distillation zone containing a high pressure' section separated from a low pressure section by a common heat transfer zone and wherein the said predominantly Ca to C4 fraction is initially fed into the high pressure section, the improvement which comprises the steps of withdrawing from a lower portion of the high pressure section a heavy residual fraction containing predominantly C3 fraction containing predominantly C2 hydrocarbons with a minor proportion of Cs hydrocarbons and sending this fraction as feed to the low pressure section; withdrawing a light liquid fraction condensed on the pressure side of the said common heat transfer zone and comprising predominantly ethylene and containing some ethane from an upper portion of the high pressure section as redux to the low pressure section; withdrawing a C: and higher hydrocarbon distillation residue stream from a lower portion of the .1 low pressure section, the Cz component of said distillation residue being predominantly ethane; and taking overhead a predominantly ethylene product stream from the low' pressure section.

and C4 hydrocarbons; withdrawing from the high pressure section an intermediate side stream 2. A process as in claim 1 in which the inter- REFERENCES CITED mediate side stream fraction from the high pres'- sure section is heat exchanged with the streams A The following references are of record in the withdrawn from the low pressure section. me of this Patent:

3. A process as in claim 2 in which a pressure 5 UNITED STATES PATENTS of about 100 p. s. i. g. is maintained in the high pressure section of the distillation zone and a. Smm Junenlatelg pressure of about 10 p. s. i. g. is maintained in the low pressure section.

JOHN A. PA'rrm'soN. 1o 

1. IN A FRACTIONATION PROCESS FOR SEPARATING ETHANE AND ETHYLENE IN A PREDOMINANTLY C2 TO C4 HYDROCARBON FRACTION IN A UNITARY DISTILLATION ZONE CONTAINING A HIGH PRESSURE SECTION SEPARATED FROM A LOW PRESSURE SECTION BY A COMMON HEAT TRANSFER ZONE AND WHEREIN THE SAID PREDOMINANTLY C2 TO C4 FRACTION IS INITIALLY FED INTO THE HIGH PRESSURE SECTION, THE IMPROVEMENT WHICH COMPRISES THE STEPS OF WITHDRAWING FROM A LOWER PORTION OF THE HIGH PRESSURE SECTION A HEAVY RESIDUAL FRACTION CONTAINING PREDOMINANTLY C3 AND C4 HYDROCARBONS; WITHDRAWING FROM THE HIGH PRESSURE SECTION AN INTERMEDIATE SIDE STREAM FRACTION CONTAINING PREDOMINANTLY C2 HYDROCARBONS WITH A MINOR PROPORTION OF C3 HYDROCARBONS AND SENDING THIS FRACTION AS FEED TO THE LOW PRESSURE SECTION; WITHDRAWING A LIGHT LIQUID FRACTION CONDENSED ON THE PRESSURE SIDE OF THE SAID COMMON HEAT TRANSFER ZONE AND COMPRISING PREDOMINANTLY ETHYLENE AND CONTAINING SOME ETHANE FROM AN UPPER PORTION OF THE HIGH PRESSURE SECTION AS REFLUX TO THE LOW PRESSURE SECTION; WITHDRAWING A C2 AND HIGHER HYDROCARBON DISTILLATION RESIDUE STREAM FROM A LOWER PORTION OF THE LOW PRESSURE SECTION, THE C2 COMPONENT OF SAID DISTILLATION RESIDUE BEING PREDOMINANTLY ETHANE; AND TAKING OVERHEAD A PREDOMINANTLY ETHYLENE PRODUCT STREAM FROM THE LOW PRESSURE SECTION. 